Portion of tooth covered by calculus or altered cementum is:

Calculus is removed by a series of controlled, overlapping, short, powerful strokes primarily using wrist-arm motion . As calculus is removed, resistance to  the  passage  of  the  cutting  edge  diminishes  until  only  a  slight roughness  remains. Longer, lighter  root-planing  strokes  are  then activated with less lateral pressure until the root surface is completely smooth and hard. The instrument handle must be rolled carefully between the thumb and fingers to keep the blade adapted closely to the tooth surface as line angles, developmental depressions, and other changes in tooth contour are followed. Scaling and root-planing strokes should be confined to the portion of the tooth where calculus or altered cementum is found; this area is known as the instrumentation zone. Sweeping the instrument over the crown where it is not needed wastes operating time, dulls the instrument, and causes loss  of control. Key Concept: Scaling and root-planing strokes should be confined to the portion of the tooth where calculus or altered cementum is found; this area is known as the instrumentation zone. Sweeping the instrument over the crown where it is not needed wastes operating time, dulls the instrument, and causes loss  of control. Ref: Newman and Carranza’s Clinical Periodontology, thirteenth edition; page no 545.e15